1POE::Kernel(3) User Contributed Perl Documentation POE::Kernel(3)
2
6 POE::Kernel - an event-based application kernel in Perl
7
9 use POE; # auto-includes POE::Kernel and POE::Session
10 POE::Session->create(
12 inline_states => {
13 _start => sub { $_[KERNEL]->yield("next") },
14 next => sub {
15 print "tick...\n";
16 $_[KERNEL]->delay(next => 1);
17 },
18 },
19 );
20 POE::Kernel->run();
22 exit;
23 In the spirit of Perl, there are a lot of other ways to use POE.
25
27 POE::Kernel is the heart of POE. It provides the lowest-level
28 features: non-blocking multiplexed I/O, timers, and signal watchers are
29 the most significant. Everything else is built upon this foundation.
30 POE::Kernel is not an event loop in itself. For that it uses one of
32 several available POE::Loop interface modules. See CPAN for modules in
33 the POE::Loop namespace.
34 POE's documentation assumes the reader understands the @_ offset
36 constants (KERNEL, HEAP, ARG0, etc.). The curious or confused reader
37 will find more detailed explanation in POE::Session.
38
40 Literally Using POE
41 POE.pm is little more than a class loader. It implements some magic to
42 cut down on the setup work.
43 Parameters to "use POE" are not treated as normal imports. Rather,
45 they're abbreviated modules to be included along with POE.
46 use POE qw(Component::Client::TCP).
48 As you can see, the leading "POE::" can be omitted this way.
50 POE.pm also includes POE::Kernel and POE::Session by default. These
52 two modules are used by nearly all POE-based programs. So the above
53 example is actually the equivalent of:
54 use POE;
56 use POE::Kernel;
57 use POE::Session;
58 use POE::Component::Client::TCP;
59 Using POE::Kernel
61 POE::Kernel needs to know which event loop you want to use. This is
62 supported in three different ways:
63 The first way is to use an event loop module before using POE::Kernel
65 (or POE, which loads POE::Kernel for you):
66 use Tk; # or one of several others
68 use POE::Kernel.
69 POE::Kernel scans the list of modules already loaded, and it loads an
71 appropriate POE::Loop adapter if it finds a known event loop.
72 The next way is to explicitly load the POE::Loop class you want:
74 use POE qw(Loop::Gtk);
76 Finally POE::Kernel's "import()" supports more programmer-friendly
78 configuration:
79 use POE::Kernel { loop => "Gtk" };
81 use POE::Session;
82 Anatomy of a POE-Based Application
84 Programs using POE work like any other. They load required modules,
85 perform some setup, run some code, and eventually exit. Halting
86 Problem notwithstanding.
87 A POE-based application loads some modules, sets up one or more
89 sessions, runs the code in those sessions, and eventually exits.
90 use POE;
92 POE::Session->create( ... map events to code here ... );
93 POE::Kernel->run();
94 exit;
95 POE::Kernel singleton
97 The POE::Kernel is a singleton object; there can be only one
98 POE::Kernel instance within a process. This allows many object methods
99 to also be package methods.
100 Sessions
102 POE implements isolated compartments called sessions. Sessions play
103 the role of tasks or threads within POE. POE::Kernel acts as POE's
104 task scheduler, doling out timeslices to each session by invoking
105 callbacks within them.
106 Callbacks are not preemptive. As long as one is running, no others
108 will be dispatched. This is known as cooperative multitasking. Each
109 session must cooperate by returning to the central dispatching kernel.
110 Cooperative multitasking vastly simplifies data sharing, since no two
112 pieces of code may alter data at once.
113 A session may also take exclusive control of a program's time, if
115 necessary, by simply not returning in a timely fashion. It's even
116 possible to write completely blocking programs that use POE as a state
117 machine rather than a cooperative dispatcher.
118 Every POE-based application needs at least one session. Code cannot
120 run within POE without being a part of some session. Likewise, a
121 threaded program always has a "thread zero".
122 Sessions in POE::Kernel should not be confused with POE::Session even
124 though the two are inextricably associated. POE::Session adapts
125 POE::Kernel's dispatcher to a particular calling convention. Other
126 POE::Session classes exist on the CPAN. Some radically alter the way
127 event handlers are called.
128 <http://search.cpan.org/search?query=poe+session>.
129 Resources
131 Resources are events and things which may create new events, such as
132 timers, I/O watchers, and even other sessions.
133 POE::Kernel tracks resources on behalf of its active sessions. It
135 generates events corresponding to these resources' activity, notifying
136 sessions when it's time to do things.
137 The conversation goes something like this:
139 Session: Be a dear, Kernel, and let me know when someone clicks on
141 this widget. Thanks so much!
142 [TIME PASSES] [SFX: MOUSE CLICK]
144 Kernel: Right, then. Someone's clicked on your widget.
146 Here you go.
147 Furthermore, since the Kernel keeps track of everything sessions do, it
149 knows when a session has run out of tasks to perform. When this
150 happens, the Kernel emits a "_stop" event at the dead session so it can
151 clean up and shutdown.
152 Kernel: Please switch off the lights and lock up; it's time to go.
154 Likewise, if a session stops on its own and there still are opened
156 resource watchers, the Kernel knows about them and cleans them up on
157 the session's behalf. POE excels at long-running services because it
158 so meticulously tracks and cleans up resources.
159 POE::Resources and the POE::Resource classes implement each kind of
161 resource, which are summarized here and covered in greater detail
162 later.
163 Events.
165 An event is a message to a sessions. Posting an event keeps both the
166 sender and the receiver alive until after the event has been
167 dispatched. This is only guaranteed if both the sender and receiver
168 are in the same process. Inter-Kernel message passing add-ons may
169 have other guarantees. Please see their documentation for details.
170 The rationale is that the event is in play, so the receiver must
172 remain active for it to be dispatched. The sender remains alive in
173 case the receiver would like to send back a response.
174 Posted events cannot be preemptively canceled. They tend to be
176 short-lived in practice, so this generally isn't an issue.
177 Timers.
179 Timers allow an application to send a message to the future. Once
180 set, a timer will keep the destination session active until it goes
181 off and the resulting event is dispatched.
182 Aliases.
184 Session aliases are an application-controlled way of addressing a
185 session. Aliases act as passive event watchers. As long as a
186 session has an alias, some other session may send events to that
187 session by that name. Aliases keep sessions alive as long as a
188 process has active sessions.
189 If the only sessions remaining are being kept alive solely by their
191 aliases, POE::Kernel will send them a terminal "IDLE" signal. In
192 most cases this will terminate the remaining sessions and allow the
193 program to exit. If the sessions remain in memory without waking up
194 on the "IDLE" signal, POE::Kernel sends them a non-maskable "ZOMBIE"
195 signal. They are then forcibly removed, and the program will finally
196 exit.
197 I/O watchers.
199 A session will remain active as long as a session is paying attention
200 to some external data source or sink. See select_read and
201 select_write.
202 Child sessions.
204 A session acting as a parent of one or more other sessions will
205 remain active until all the child sessions stop. This may be
206 bypassed by detaching the children from the parent.
207 Child processes.
209 Child process are watched by sig_child(). The sig_child() watcher
210 will keep the watching session active until the child process has
211 been reaped by POE::Kernel and the resulting event has been
212 dispatched.
213 All other signal watchers, including using "sig" to watch for "CHLD",
215 do not keep their sessions active. If you need a session to remain
216 active when it's only watching for signals, have it set an alias or
217 one of its own public reference counters.
218 Public reference counters.
220 A session will remain active as long as it has one or more nonzero
221 public (or external) reference counter.
222 Session Lifespans
224 "Session" as a term is somewhat overloaded. There are two related
225 concepts that share the name. First there is the class POE::Session,
226 and objects created with it or related classes. Second there is a data
227 structure within POE::Kernel that tracks the POE::Session objects in
228 play and the various resources owned by each.
229 The way POE's garbage collector works is that a session object gives
231 itself to POE::Kernel at creation time. The Kernel then holds onto
232 that object as long as resources exist that require the session to
233 remain alive. When all of these resources are destroyed or released,
234 the session object has nothing left to trigger activity. POE::Kernel
235 notifies the object it's through, and cleans up its internal session
236 context. The session object is released, and self-destructs in the
237 normal Perlish fashion.
238 Sessions may be stopped even if they have active resources. For
240 example, a session may fail to handle a terminal signal. In this case,
241 POE::Kernel forces the session to stop, and all resources associated
242 with the session are preemptively released.
243 Events
245 An event is a message that is sent from one part of the POE application
246 to another. An event consists of the event's name, optional event-
247 specific parameters and OOB information. An event may be sent from the
248 kernel, from a wheel or from a session.
249 An application creates an event with "post", "yield", "call" or even
251 "signal". POE::Kernel creates events in response external stimulus
252 (signals, select, etc).
253 Event Handlers
255 An event is handled by a function called an event handler, which is
257 some code that is designated to be called when a particular event is
258 dispatched. See "Event Handler Management" and POE::Session.
259 The term state is often used in place of event handler, especially when
261 treating sessions as event driven state machines.
262 Handlers are always called in scalar context for asynchronous events
264 (i.e. via post()). Synchronous events, invoked with call(), are
265 handled in the same context that call() was called.
266 Event handlers may not directly return references to objects in the
268 "POE" namespace. POE::Kernel will stringify these references to
269 prevent timing issues with certain objects' destruction. For example,
270 this error handler would cause errors because a deleted wheel would not
271 be destructed when one might think:
272 sub handle_error {
274 warn "Got an error";
275 delete $_[HEAP]{wheel};
276 }
277 The delete() call returns the deleted wheel member, which is then
279 returned implicitly by handle_error().
280 Using POE with Other Event Loops
282 POE::Kernel supports any number of event loops. Two are included in
283 the base distribution. Historically, POE included other loops but they
284 were moved into a separate distribution. You can find them and other
285 loops on the CPAN.
286 POE's public interfaces remain the same regardless of the event loop
288 being used. Since most graphical toolkits include some form of event
289 loop, back-end code should be portable to all of them.
290 POE's cooperation with other event loops lets POE be embedded into
292 other software. The common underlying event loop drives both the
293 application and POE. For example, by using POE::Loop::Glib, one can
294 embed POE into Vim, irssi, and so on. Application scripts can then
295 take advantage of POE::Component::Client::HTTP (and everything else) to
296 do large-scale work without blocking the rest of the program.
297 Because this is Perl, there are multiple ways to load an alternate
299 event loop. The simplest way is to load the event loop before loading
300 POE::Kernel.
301 use Gtk;
303 use POE;
304 Remember that POE loads POE::Kernel internally.
306 POE::Kernel examines the modules loaded before it and detects that Gtk
308 has been loaded. If POE::Loop::Gtk is available, POE loads and hooks
309 it into POE::Kernel automatically.
310 It's less mysterious to load the appropriate POE::Loop class directly.
312 Their names follow the format "POE::Loop::$loop_module_name", where
313 $loop_module_name is the name of the event loop module after each "::"
314 has been substituted with an underscore. It can be abbreviated using
315 POE's loader magic.
316 use POE qw( Loop::Event_Lib );
318 POE also recognizes XS loops, they reside in the
320 "POE::XS::Loop::$loop_module_name" namespace. Using them may give you
321 a performance improvement on your platform, as the eventloop are some
322 of the hottest code in the system. As always, benchmark your
323 application against various loops to see which one is best for your
324 workload and platform.
325 use POE qw( XS::Loop::EPoll );
327 Please don't load the loop modules directly, because POE will not have
329 a chance to initialize it's internal structures yet. Code written like
330 this will throw errors on startup. It might look like a bug in POE, but
331 it's just the way POE is designed.
332 use POE::Loop::IO_Poll;
334 use POE;
335 POE::Kernel also supports configuration directives on its own "use"
337 line. A loop explicitly specified this way will override the search
338 logic.
339 use POE::Kernel { loop => "Glib" };
341 Finally, one may specify the loop class by setting the POE::Loop or
343 POE::XS:Loop class name in the POE_EVENT_LOOP environment variable.
344 This mechanism was added for tests that need to specify the loop from a
345 distance.
346 BEGIN { $ENV{POE_EVENT_LOOP} = "POE::XS::Loop::Poll" }
348 use POE;
349 Of course this may also be set from your shell:
351 % export POE_EVENT_LOOP='POE::XS::Loop::Poll'
353 % make test
354 Many external event loops support their own callback mechanisms.
356 POE::Session's "postback()" and "callback()" methods return plain Perl
357 code references that will generate POE events when called.
358 Applications can pass these code references to event loops for use as
359 callbacks.
360 POE's distribution includes two event loop interfaces. CPAN holds
362 several more:
363 POE::Loop::Select (bundled)
365 By default POE uses its select() based loop to drive its event system.
367 This is perhaps the least efficient loop, but it is also the most
368 portable. POE optimizes for correctness above all.
369 POE::Loop::IO_Poll (bundled)
371 The IO::Poll event loop provides an alternative that theoretically
373 scales better than select().
374 POE::Loop::Event (separate distribution)
376 This event loop provides interoperability with other modules that use
378 Event. It may also provide a performance boost because Event is
379 written in a compiled language. Unfortunately, this makes Event less
380 portable than Perl's built-in select().
381 POE::Loop::Gtk (separate distribution)
383 This event loop allows programs to work under the Gtk graphical
385 toolkit.
386 POE::Loop::Tk (separate distribution)
388 This event loop allows programs to work under the Tk graphical toolkit.
390 Tk has some restrictions that require POE to behave oddly.
391 Tk's event loop will not run unless one or more widgets are created.
393 POE must therefore create such a widget before it can run. POE::Kernel
394 exports $poe_main_window so that the application developer may use the
395 widget (which is a MainWindow), since POE doesn't need it other than
396 for dispatching events.
397 Creating and using a different MainWindow often has an undesired
399 outcome.
400 POE::Loop::EV (separate distribution)
402 POE::Loop::EV allows POE-based programs to use the EV event library
404 with little or no change.
405 POE::Loop::Glib (separate distribution)
407 POE::Loop::Glib allows POE-based programs to use Glib with little or no
409 change. It also supports embedding POE-based programs into
410 applications that already use Glib. For example, we have heard that
411 POE has successfully embedded into vim, irssi and xchat via this loop.
412 POE::Loop::Kqueue (separate distribution)
414 POE::Loop::Kqueue allows POE-based programs to transparently use the
416 BSD kqueue event library on operating systems that support it.
417 POE::Loop::Prima (separate distribution)
419 POE::Loop::Prima allows POE-based programs to use Prima's event loop
421 with little or no change. It allows POE libraries to be used within
422 Prima applications.
423 POE::Loop::Wx (separate distribution)
425 POE::Loop::Wx allows POE-based programs to use Wx's event loop with
427 little or no change. It allows POE libraries to be used within Wx
428 applications, such as Padre.
429 POE::XS::Loop::EPoll (separate distribution)
431 POE::XS::Loop::EPoll allows POE components to transparently use the
433 EPoll event library on operating systems that support it.
434 POE::XS::Loop::Poll (separate distribution)
436 POE::XS::Loop::Poll is a higher-performance C-based libpoll event loop.
438 It replaces some of POE's hot Perl code with C for better performance.
439 Other Event Loops (separate distributions)
441 POE may be extended to handle other event loops. Developers are
443 invited to work with us to support their favorite loops.
444
446 POE::Kernel encapsulates a lot of features. The documentation for each
447 set of features is grouped by purpose.
448 Kernel Management and Accessors
450 ID
451 ID() currently returns POE::Kernel's unique identifier. Every Kernel
453 instance is assigned a globally unique ID at birth. has_forked()
454 alters the ID so that each forked process has a unique one, too.
455 % perl -wl -MPOE -e 'print $poe_kernel->ID'
457 macbookpoe.local-4d5305de-0000e6b8-00000001
458 The content of these IDs may change from time to time. Your code
460 should not depend upon the current format.
461 Deprecation Warning 2011-02-09
463 Your code should not depend upon ID() remaining unique. The uniqueness
465 will be removed in a future release of POE. If you require unique IDs,
466 please see one of the fine GUID and/or UUID modules on the CPAN:
467 http://search.cpan.org/search?query=GUID&mode=dist
469 http://search.cpan.org/search?query=UUID&mode=dist
470 POE doesn't require globally or universally unique kernel IDs. The
472 creation and maintenance of these IDs adds overhead to POE::Kernel's
473 has_forked() method. Other modules do it better, upon demand, without
474 incurring overhead for those who don't need them.
475 run
477 run() runs POE::Kernel's event dispatcher. It will not return until
479 all sessions have ended. run() is a class method so a POE::Kernel
480 reference is not needed to start a program's execution.
481 use POE;
483 POE::Session->create( ... ); # one or more
484 POE::Kernel->run(); # set them all running
485 exit;
486 POE implements the Reactor pattern at its core. Events are dispatched
488 to functions and methods through callbacks. The code behind run()
489 waits for and dispatches events.
490 run() will not return until every session has ended. This includes
492 sessions that were created while run() was running.
493 POE::Kernel will print a strong message if a program creates sessions
495 but fails to call run(). Prior to this warning, we received tons of
496 bug reports along the lines of "my POE program isn't doing anything".
497 It turned out that people forgot to start an event dispatcher, so
498 events were never dispatched.
499 If the lack of a run() call is deliberate, perhaps because some other
501 event loop already has control, you can avoid the message by calling it
502 before creating a session. run() at that point will initialize POE and
503 return immediately. POE::Kernel will be satisfied that run() was
504 called, although POE will not have actually taken control of the event
505 loop.
506 use POE;
508 POE::Kernel->run(); # silence the warning
509 POE::Session->create( ... );
510 exit;
511 Note, however, that this varies from one event loop to another. If a
513 particular POE::Loop implementation doesn't support it, that's probably
514 a bug. Please file a bug report with the owner of the relevant
515 POE::Loop module.
516 run_one_timeslice
518 run_one_timeslice() dispatches any events that are due to be delivered.
520 These events include timers that are due, asynchronous messages that
521 need to be delivered, signals that require handling, and notifications
522 for files with pending I/O. Do not rely too much on event ordering.
523 run_one_timeslice() is defined by the underlying event loop, and its
524 timing may vary.
525 run() is implemented similar to
527 run_one_timeslice() while $session_count > 0;
529 run_one_timeslice() can be used to keep running POE::Kernel's
531 dispatcher while emulating blocking behavior. The pattern is
532 implemented with a flag that is set when some asynchronous event
533 occurs. A loop calls run_one_timeslice() until that flag is set. For
534 example:
535 my $done = 0;
537 sub handle_some_event {
539 $done = 1;
540 }
541 $kernel->run_one_timeslice() while not $done;
543 Do be careful. The above example will spin if POE::Kernel is done but
545 $done is never set. The loop will never be done, even though there's
546 nothing left that will set $done.
547 run_while SCALAR_REF
549 run_while() is an experimental version of run_one_timeslice() that will
551 only return when there are no more active sessions, or the value of the
552 referenced scalar becomes false.
553 Here's a version of the run_one_timeslice() example using run_while()
555 instead:
556 my $job_count = 3;
558 sub handle_some_event {
560 $job_count--;
561 }
562 $kernel->run_while(\$job_count);
564 has_forked
566 my $pid = fork();
568 die "Unable to fork" unless defined $pid;
569 unless( $pid ) {
570 $poe_kernel->has_forked;
571 }
572 Inform the kernel that it is now running in a new process. This allows
574 the kernel to reset some internal data to adjust to the new situation.
575 has_forked() must be called in the child process if you wish to run the
577 same kernel. However, if you want the child process to have new
578 kernel, you must call "stop" instead.
579 Note: POE's internals will detect if a fork occurred before "run()" and
581 will call "has_forked()" automatically. If you are unsure whether you
582 need to call it or not, please enable "ASSERT_USAGE" and POE will emit
583 a warning if it's necessary.
584 stop
586 stop() causes POE::Kernel->run() to return early. It does this by
588 emptying the event queue, freeing all used resources, and stopping
589 every active session. stop() is not meant to be used lightly. Proceed
590 with caution.
591 Caveats:
593 The session that calls stop() will not be fully DESTROYed until it
595 returns. Invoking an event handler in the session requires a reference
596 to that session, and weak references are prohibited in POE for backward
597 compatibility reasons, so it makes sense that the last session won't be
598 garbage collected right away.
599 Sessions are not notified about their destruction. If anything relies
601 on _stop being delivered, it will break and/or leak memory.
602 stop() is still considered experimental. It was added to improve
604 fork() support for POE::Wheel::Run. If it proves unfixably
605 problematic, it will be removed without much notice.
606 stop() is advanced magic. Programmers who think they need it are
608 invited to become familiar with its source.
609 See "Running POE::Kernel in the Child" in POE::Wheel::Run for an
611 example of how to use this facility.
612 Asynchronous Messages (FIFO Events)
614 Asynchronous messages are events that are dispatched in the order in
615 which they were enqueued (the first one in is the first one out,
616 otherwise known as first-in/first-out, or FIFO order). These methods
617 enqueue new messages for delivery. The act of enqueuing a message
618 keeps the sender alive at least until the message is delivered.
619 post DESTINATION, EVENT_NAME [, PARAMETER_LIST]
621 post() enqueues a message to be dispatched to a particular DESTINATION
623 session. The message will be handled by the code associated with
624 EVENT_NAME. If a PARAMETER_LIST is included, its values will also be
625 passed along.
626 POE::Session->create(
628 inline_states => {
629 _start => sub {
630 $_[KERNEL]->post( $_[SESSION], "event_name", 0 );
631 },
632 event_name => sub {
633 print "$_[ARG0]\n";
634 $_[KERNEL]->post( $_[SESSION], "event_name", $_[ARG0] + 1 );
635 },
636 }
637 );
638 post() returns a Boolean value indicating whether the message was
640 successfully enqueued. If post() returns false, $! is set to explain
641 the failure:
642 ESRCH ("No such process") - The DESTINATION session did not exist at
644 the time post() was called.
645 yield EVENT_NAME [, PARAMETER_LIST]
647 yield() is a shortcut for post() where the destination session is the
649 same as the sender. This example is equivalent to the one for post():
650 POE::Session->create(
652 inline_states => {
653 _start => sub {
654 $_[KERNEL]->yield( "event_name", 0 );
655 },
656 event_name => sub {
657 print "$_[ARG0]\n";
658 $_[KERNEL]->yield( "event_name", $_[ARG0] + 1 );
659 },
660 }
661 );
662 As with post(), yield() returns right away, and the enqueued EVENT_NAME
664 is dispatched later. This may be confusing if you're already familiar
665 with threading.
666 yield() should always succeed, so it does not return a meaningful
668 value.
669 Synchronous Messages
671 It is sometimes necessary for code to be invoked right away. For
672 example, some resources must be serviced right away, or they'll
673 faithfully continue reporting their readiness. These reports would
674 appear as a stream of duplicate events. Synchronous events can also
675 prevent data from going stale between the time an event is enqueued and
676 the time it's delivered.
677 Synchronous event handlers preempt POE's event queue, so they should
679 perform simple tasks of limited duration. Synchronous events that need
680 to do more than just service a resource should pass the resource's
681 information to an asynchronous handler. Otherwise synchronous
682 operations will occur out of order in relation to asynchronous events.
683 It's very easy to have race conditions or break causality this way, so
684 try to avoid it unless you're okay with the consequences.
685 POE provides these ways to call message handlers right away.
687 call DESTINATION, EVENT_NAME [, PARAMETER_LIST]
689 call()'s semantics are nearly identical to post()'s. call() invokes a
691 DESTINATION's handler associated with an EVENT_NAME. An optional
692 PARAMETER_LIST will be passed along to the message's handler. The
693 difference, however, is that the handler will be invoked immediately,
694 even before call() returns.
695 call() returns the value returned by the EVENT_NAME handler. It can do
697 this because the handler is invoked before call() returns. call() can
698 therefore be used as an accessor, although there are better ways to
699 accomplish simple accessor behavior.
700 POE::Session->create(
702 inline_states => {
703 _start => sub {
704 print "Got: ", $_[KERNEL]->call($_[SESSION], "do_now"), "\n";
705 },
706 do_now => sub {
707 return "some value";
708 }
709 }
710 );
711 The POE::Wheel classes uses call() to synchronously deliver I/O
713 notifications. This avoids a host of race conditions.
714 call() may fail in the same way and for the same reasons as post(). On
716 failure, $! is set to some nonzero value indicating why. Since call()
717 may return undef as a matter of course, it's recommended that $! be
718 checked for the error condition as well as the explanation.
719 ESRCH ("No such process") - The DESTINATION session did not exist at
721 the time post() was called.
722 Timer Events (Delayed Messages)
724 It's often useful to wait for a certain time or until a certain amount
725 of time has passed. POE supports this with events that are deferred
726 until either an absolute time ("alarms") or until a certain duration of
727 time has elapsed ("delays").
728 Timer interfaces are further divided into two groups. One group
730 identifies timers by the names of their associated events. Another
731 group identifies timers by a unique identifier returned by the timer
732 constructors. Technically, the two are both name-based, but the
733 "identifier-based" timers provide a second, more specific handle to
734 identify individual timers.
735 Timers may only be set up for the current session. This design was
737 modeled after alarm() and SIGALRM, which only affect the current UNIX
738 process. Each session has a separate namespace for timer names. Timer
739 methods called in one session cannot affect the timers in another. As
740 you may have noticed, quite a lot of POE's API is designed to prevent
741 sessions from interfering with each other.
742 The best way to simulate deferred inter-session messages is to send an
744 immediate message that causes the destination to set a timer. The
745 destination's timer then defers the action requested of it. This way
746 is preferred because the time spent communicating the request between
747 sessions may not be trivial, especially if the sessions are separated
748 by a network. The destination can determine how much time remains on
749 the requested timer and adjust its wait time accordingly.
750 Name-Based Timers
752 Name-based timers are identified by the event names used to set them.
754 It is possible for different sessions to use the same timer event
755 names, since each session is a separate compartment with its own timer
756 namespace. It is possible for a session to have multiple timers for a
757 given event, but results may be surprising. Be careful to use the
758 right timer methods.
759 The name-based timer methods are alarm(), alarm_add(), delay(), and
761 delay_add().
762 alarm EVENT_NAME [, EPOCH_TIME [, PARAMETER_LIST] ]
764 alarm() clears all existing timers in the current session with the same
766 EVENT_NAME. It then sets a new timer, named EVENT_NAME, that will fire
767 EVENT_NAME at the current session when EPOCH_TIME has been reached. An
768 optional PARAMETER_LIST may be passed along to the timer's handler.
769 Omitting the EPOCH_TIME and subsequent parameters causes alarm() to
771 clear the EVENT_NAME timers in the current session without setting a
772 new one.
773 EPOCH_TIME is the UNIX epoch time. You know, seconds since midnight,
775 1970-01-01. POE uses Time::HiRes::time(), which allows EPOCH_TIME to
776 be (or include) fractional seconds.
777 POE supports fractional seconds, but accuracy falls off steeply after
779 1/100 second. Mileage will vary depending on your CPU speed and your
780 OS time resolution.
781 Be sure to use Time::HiRes::time() rather than Perl's built-in time()
783 if sub-second accuracy matters at all. The built-in time() returns
784 floor(Time::HiRes::time()), which is nearly always some fraction of a
785 second in the past. For example the high-resolution time might be
786 1200941422.89996. At that same instant, time() would be 1200941422.
787 An alarm for time() + 0.5 would be 0.39996 seconds in the past, so it
788 would be dispatched immediately (if not sooner).
789 POE's event queue is time-ordered, so a timer due before time() will be
791 delivered ahead of other events but not before timers with even earlier
792 due times. Therefore an alarm() with an EPOCH_TIME before time() jumps
793 ahead of the queue.
794 All timers are implemented identically internally, regardless of how
796 they are set. alarm() will therefore blithely clear timers set by
797 other means.
798 POE::Session->create(
800 inline_states => {
801 _start => sub {
802 $_[KERNEL]->alarm( tick => time() + 1, 0 );
803 },
804 tick => sub {
805 print "tick $_[ARG0]\n";
806 $_[KERNEL]->alarm( tock => time() + 1, $_[ARG0] + 1 );
807 },
808 tock => sub {
809 print "tock $_[ARG0]\n";
810 $_[KERNEL]->alarm( tick => time() + 1, $_[ARG0] + 1 );
811 },
812 }
813 );
814 alarm() returns 0 on success or a true value on failure. Usually
816 EINVAL to signal an invalid parameter, such as an undefined EVENT_NAME.
817 alarm_add EVENT_NAME, EPOCH_TIME [, PARAMETER_LIST]
819 alarm_add() is used to add a new alarm timer named EVENT_NAME without
821 clearing existing timers. EPOCH_TIME is a required parameter.
822 Otherwise the semantics are identical to alarm().
823 A program may use alarm_add() without first using alarm().
825 POE::Session->create(
827 inline_states => {
828 _start => sub {
829 $_[KERNEL]->alarm_add( tick => time() + 1.0, 1_000_000 );
830 $_[KERNEL]->alarm_add( tick => time() + 1.5, 2_000_000 );
831 },
832 tick => sub {
833 print "tick $_[ARG0]\n";
834 $_[KERNEL]->alarm_add( tock => time() + 1, $_[ARG0] + 1 );
835 },
836 tock => sub {
837 print "tock $_[ARG0]\n";
838 $_[KERNEL]->alarm_add( tick => time() + 1, $_[ARG0] + 1 );
839 },
840 }
841 );
842 alarm_add() returns 0 on success or EINVAL if EVENT_NAME or EPOCH_TIME
844 is undefined.
845 delay EVENT_NAME [, DURATION_SECONDS [, PARAMETER_LIST] ]
847 delay() clears all existing timers in the current session with the same
849 EVENT_NAME. It then sets a new timer, named EVENT_NAME, that will fire
850 EVENT_NAME at the current session when DURATION_SECONDS have elapsed
851 from "now". An optional PARAMETER_LIST may be passed along to the
852 timer's handler.
853 Omitting the DURATION_SECONDS and subsequent parameters causes delay()
855 to clear the EVENT_NAME timers in the current session without setting a
856 new one.
857 DURATION_SECONDS may be or include fractional seconds. As with all of
859 POE's timers, accuracy falls off steeply after 1/100 second. Mileage
860 will vary depending on your CPU speed and your OS time resolution.
861 POE's event queue is time-ordered, so a timer due before time() will be
863 delivered ahead of other events but not before timers with even earlier
864 due times. Therefore a delay () with a zero or negative
865 DURATION_SECONDS jumps ahead of the queue.
866 delay() may be considered a shorthand form of alarm(), but there are
868 subtle differences in timing issues. This code is roughly equivalent
869 to the alarm() example.
870 POE::Session->create(
872 inline_states => {
873 _start => sub {
874 $_[KERNEL]->delay( tick => 1, 0 );
875 },
876 tick => sub {
877 print "tick $_[ARG0]\n";
878 $_[KERNEL]->delay( tock => 1, $_[ARG0] + 1 );
879 },
880 tock => sub {
881 print "tock $_[ARG0]\n";
882 $_[KERNEL]->delay( tick => 1, $_[ARG0] + 1 );
883 },
884 }
885 );
886 delay() returns 0 on success or a reason for failure: EINVAL if
888 EVENT_NAME is undefined.
889 delay_add EVENT_NAME, DURATION_SECONDS [, PARAMETER_LIST]
891 delay_add() is used to add a new delay timer named EVENT_NAME without
893 clearing existing timers. DURATION_SECONDS is a required parameter.
894 Otherwise the semantics are identical to delay().
895 A program may use delay_add() without first using delay().
897 POE::Session->create(
899 inline_states => {
900 _start => sub {
901 $_[KERNEL]->delay_add( tick => 1.0, 1_000_000 );
902 $_[KERNEL]->delay_add( tick => 1.5, 2_000_000 );
903 },
904 tick => sub {
905 print "tick $_[ARG0]\n";
906 $_[KERNEL]->delay_add( tock => 1, $_[ARG0] + 1 );
907 },
908 tock => sub {
909 print "tock $_[ARG0]\n";
910 $_[KERNEL]->delay_add( tick => 1, $_[ARG0] + 1 );
911 },
912 }
913 );
914 delay_add() returns 0 on success or EINVAL if EVENT_NAME or EPOCH_TIME
916 is undefined.
917 Identifier-Based Timers
919 A second way to manage timers is through identifiers. Setting an alarm
921 or delay with the "identifier" methods allows a program to manipulate
922 several timers with the same name in the same session. As covered in
923 alarm() and delay() however, it's possible to mix named and identified
924 timer calls, but the consequences may not always be expected.
925 alarm_set EVENT_NAME, EPOCH_TIME [, PARAMETER_LIST]
927 alarm_set() sets an alarm, returning a unique identifier that can be
929 used to adjust or remove the alarm later. Unlike alarm(), it does not
930 first clear existing timers with the same EVENT_NAME. Otherwise the
931 semantics are identical to alarm().
932 POE::Session->create(
934 inline_states => {
935 _start => sub {
936 $_[HEAP]{alarm_id} = $_[KERNEL]->alarm_set(
937 party => time() + 1999
938 );
939 $_[KERNEL]->delay(raid => 1);
940 },
941 raid => sub {
942 $_[KERNEL]->alarm_remove( delete $_[HEAP]{alarm_id} );
943 },
944 }
945 );
946 alarm_set() returns false if it fails and sets $! with the explanation.
948 $! will be EINVAL if EVENT_NAME or TIME is undefined.
949 alarm_adjust ALARM_ID, DELTA_SECONDS
951 alarm_adjust() adjusts an existing timer's due time by DELTA_SECONDS,
953 which may be positive or negative. It may even be zero, but that's not
954 as useful. On success, it returns the timer's new due time since the
955 start of the UNIX epoch.
956 It's possible to alarm_adjust() timers created by delay_set() as well
958 as alarm_set().
959 This example moves an alarm's due time ten seconds earlier.
961 use POSIX qw(strftime);
963 POE::Session->create(
965 inline_states => {
966 _start => sub {
967 $_[HEAP]{alarm_id} = $_[KERNEL]->alarm_set(
968 party => time() + 1999
969 );
970 $_[KERNEL]->delay(postpone => 1);
971 },
972 postpone => sub {
973 my $new_time = $_[KERNEL]->alarm_adjust(
974 $_[HEAP]{alarm_id}, -10
975 );
976 print(
977 "Now we're gonna party like it's ",
978 strftime("%F %T", gmtime($new_time)), "\n"
979 );
980 },
981 }
982 );
983 alarm_adjust() returns Boolean false if it fails, setting $! to the
985 reason why. $! may be EINVAL if ALARM_ID or DELTA_SECONDS are
986 undefined. It may be ESRCH if ALARM_ID no longer refers to a pending
987 timer. $! may also contain EPERM if ALARM_ID is valid but belongs to a
988 different session.
989 alarm_remove ALARM_ID
991 alarm_remove() removes the alarm identified by ALARM_ID. ALARM_ID
993 comes from a previous alarm_set() or delay_set() call.
994 Upon success, alarm_remove() returns something true based on its
996 context. In a list context, it returns three things: The removed
997 alarm's event name, the UNIX time it was due to go off, and a reference
998 to the PARAMETER_LIST (if any) assigned to the timer when it was
999 created. If necessary, the timer can be re-set with this information.
1000 POE::Session->create(
1002 inline_states => {
1003 _start => sub {
1004 $_[HEAP]{alarm_id} = $_[KERNEL]->alarm_set(
1005 party => time() + 1999
1006 );
1007 $_[KERNEL]->delay(raid => 1);
1008 },
1009 raid => sub {
1010 my ($name, $time, $param) = $_[KERNEL]->alarm_remove(
1011 $_[HEAP]{alarm_id}
1012 );
1013 print(
1014 "Removed alarm for event $name due at $time with @$param\n"
1015 );
1016 # Or reset it, if you'd like. Possibly after modification.
1018 $_[KERNEL]->alarm_set($name, $time, @$param);
1019 },
1020 }
1021 );
1022 In a scalar context, it returns a reference to a list of the three
1024 things above.
1025 # Remove and reset an alarm.
1027 my $alarm_info = $_[KERNEL]->alarm_remove( $alarm_id );
1028 my $new_id = $_[KERNEL]->alarm_set(
1029 $alarm_info[0], $alarm_info[1], @{$alarm_info[2]}
1030 );
1031 Upon failure, however, alarm_remove() returns a Boolean false value and
1033 sets $! with the reason why the call failed:
1034 EINVAL ("Invalid argument") indicates a problem with one or more
1036 parameters, usually an undefined ALARM_ID.
1037 ESRCH ("No such process") indicates that ALARM_ID did not refer to a
1039 pending alarm.
1040 EPERM ("Operation not permitted"). A session cannot remove an alarm it
1042 does not own.
1043 alarm_remove_all
1045 alarm_remove_all() removes all the pending timers for the current
1047 session, regardless of creation method or type. This method takes no
1048 arguments. It returns information about the alarms that were removed,
1049 either as a list of alarms or a list reference depending whether
1050 alarm_remove_all() is called in scalar or list context.
1051 Each removed alarm's information is identical to the format explained
1053 in alarm_remove().
1054 sub some_event_handler {
1056 my @removed_alarms = $_[KERNEL]->alarm_remove_all();
1057 foreach my $alarm (@removed_alarms) {
1058 my ($name, $time, $param) = @$alarm;
1059 ...;
1060 }
1061 }
1062 delay_set EVENT_NAME, DURATION_SECONDS [, PARAMETER_LIST]
1064 delay_set() sets a timer for DURATION_SECONDS in the future. The timer
1066 will be dispatched to the code associated with EVENT_NAME in the
1067 current session. An optional PARAMETER_LIST will be passed through to
1068 the handler. It returns the same sort of things that alarm_set() does.
1069 POE::Session->create(
1071 inline_states => {
1072 _start => sub {
1073 $_[KERNEL]->delay_set("later", 5, "hello", "world");
1074 },
1075 later => sub {
1076 print "@_[ARG0..#$_]\n";
1077 }
1078 }
1079 );
1080 delay_adjust ALARM_ID, SECONDS_FROM_NOW
1082 delay_adjust() changes a timer's due time to be SECONDS_FROM_NOW. It's
1084 useful for refreshing watchdog- or timeout-style timers. On success it
1085 returns the new absolute UNIX time the timer will be due.
1086 It's possible for delay_adjust() to adjust timers created by
1088 alarm_set() as well as delay_set().
1089 use POSIX qw(strftime);
1091 POE::Session->create(
1093 inline_states => {
1094 # Setup.
1095 # ... omitted.
1096 got_input => sub {
1098 my $new_time = $_[KERNEL]->delay_adjust(
1099 $_[HEAP]{input_timeout}, 60
1100 );
1101 print(
1102 "Refreshed the input timeout. Next may occur at ",
1103 strftime("%F %T", gmtime($new_time)), "\n"
1104 );
1105 },
1106 }
1107 );
1108 On failure it returns Boolean false and sets $! to a reason for the
1110 failure. See the explanation of $! for alarm_adjust().
1111 delay_remove is not needed
1113 There is no delay_remove(). Timers are all identical internally, so
1115 alarm_remove() will work with timer IDs returned by delay_set().
1116 delay_remove_all is not needed
1118 There is no delay_remove_all(). Timers are all identical internally,
1120 so alarm_remove_all() clears them all regardless how they were created.
1121 Comparison
1123 Below is a table to help compare the various delayed message-sending
1125 methods
1126 +-----------+------------------+---------------------+------------+
1128 | | time argument | clears other events | returns on |
1129 | method | passed to method | of the same name | success |
1130 +-----------+------------------+---------------------+------------+
1131 | delay_set | seconds from now | N | alarm_id |
1132 | delay | seconds from now | Y | 0 (false) |
1133 | alarm_set | unix epoch time | N | alarm_id |
1134 | alarm | unix epoch time | Y | 0 (false) |
1135 +-----------+------------------+---------------------+------------+
1136 Session Identifiers (IDs and Aliases)
1138 A session may be referred to by its object references (either blessed
1139 or stringified), a session ID, or one or more symbolic names we call
1140 aliases.
1141 Every session is represented by an object, so session references are
1143 fairly straightforward. POE::Kernel may reference these objects. For
1144 instance, post() may use $_[SENDER] as a destination:
1145 POE::Session->create(
1147 inline_states => {
1148 _start => sub { $_[KERNEL]->alias_set("echoer") },
1149 ping => sub {
1150 $_[KERNEL]->post( $_[SENDER], "pong", @_[ARG0..$#_] );
1151 }
1152 }
1153 );
1154 POE also recognized stringified Session objects for convenience and as
1156 a form of weak reference. Here $_[SENDER] is wrapped in quotes to
1157 stringify it:
1158 POE::Session->create(
1160 inline_states => {
1161 _start => sub { $_[KERNEL]->alias_set("echoer") },
1162 ping => sub {
1163 $_[KERNEL]->post( "$_[SENDER]", "pong", @_[ARG0..$#_] );
1164 }
1165 }
1166 );
1167 Every session is assigned a unique ID at creation time. No two active
1169 sessions will have the same ID, but IDs may be reused over time. The
1170 combination of a kernel ID and a session ID should be sufficient as a
1171 global unique identifier.
1172 POE::Session->create(
1174 inline_states => {
1175 _start => sub { $_[KERNEL]->alias_set("echoer") },
1176 ping => sub {
1177 $_[KERNEL]->delay(
1178 pong_later => rand(5), $_[SENDER]->ID, @_[ARG0..$#_]
1179 );
1180 },
1181 pong_later => sub {
1182 $_[KERNEL]->post( $_[ARG0], "pong", @_[ARG1..$#_] );
1183 }
1184 }
1185 );
1186 Kernels also maintain a global session namespace or dictionary from
1188 which may be used to map a symbolic aliases to a session. Once an alias
1189 is mapping has been created, that alias may be used to refer to the
1190 session wherever a session may be specified.
1191 In the previous examples, each echoer service has set an "echoer"
1193 alias. Another session can post a ping request to the echoer session
1194 by using that alias rather than a session object or ID. For example:
1195 POE::Session->create(
1197 inline_states => {
1198 _start => sub { $_[KERNEL]->post(echoer => ping => "whee!" ) },
1199 pong => sub { print "@_[ARG0..$#_]\n" }
1200 }
1201 );
1202 A session with an alias will not stop until all other activity has
1204 stopped. Aliases are treated as a kind of event watcher. Events come
1205 from active sessions. Aliases therefore become useless when there are
1206 no active sessions left. Rather than leaving the program running in a
1207 "zombie" state, POE detects this deadlock condition and triggers a
1208 cleanup. See "Signal Classes" for more information.
1209 alias_set ALIAS
1211 alias_set() maps an ALIAS in POE::Kernel's dictionary to the current
1213 session. The ALIAS may then be used nearly everywhere a session
1214 reference, stringified reference, or ID is expected.
1215 Sessions may have more than one alias. Each alias must be defined in a
1217 separate alias_set() call. A single alias may not refer to more than
1218 one session.
1219 Multiple alias examples are above.
1221 alias_set() returns 0 on success, or a nonzero failure indicator:
1223 EEXIST ("File exists") indicates that the alias is already assigned to
1224 to a different session.
1225 alias_remove ALIAS
1227 alias_remove() removes an ALIAS for the current session from
1229 POE::Kernel's dictionary. The ALIAS will no longer refer to the
1230 current session. This does not negatively affect events already posted
1231 to POE's queue. Alias resolution occurs at post() time, not at
1232 delivery time.
1233 POE::Session->create(
1235 inline_states => {
1236 _start => sub {
1237 $_[KERNEL]->alias_set("short_window");
1238 $_[KERNEL]->delay(close_window => 1);
1239 },
1240 close_window => {
1241 $_[KERNEL]->alias_remove("short_window");
1242 }
1243 }
1244 );
1245 alias_remove() returns 0 on success or a nonzero failure code: ESRCH
1247 ("No such process") indicates that the ALIAS is not currently in
1248 POE::Kernel's dictionary. EPERM ("Operation not permitted") means that
1249 the current session may not remove the ALIAS because it is in use by
1250 some other session.
1251 alias_resolve ALIAS
1253 alias_resolve() returns a session reference corresponding to a given
1255 ALIAS. Actually, the ALIAS may be a stringified session reference, a
1256 session ID, or an alias previously registered by alias_set().
1257 One use for alias_resolve() is to detect whether another session has
1259 gone away:
1260 unless (defined $_[KERNEL]->alias_resolve("Elvis")) {
1262 print "Elvis has left the building.\n";
1263 }
1264 As previously mentioned, alias_resolve() returns a session reference or
1266 undef on failure. Failure also sets $! to ESRCH ("No such process")
1267 when the ALIAS is not currently in POE::Kernel's.
1268 alias_list [SESSION_REFERENCE]
1270 alias_list() returns a list of aliases associated with a specific
1272 SESSION, or with the current session if SESSION is omitted.
1273 alias_list() returns an empty list if the requested SESSION has no
1274 aliases.
1275 SESSION may be a session reference (blessed or stringified), a session
1277 ID, or a session alias.
1278 POE::Session->create(
1280 inline_states => {
1281 $_[KERNEL]->alias_set("mi");
1282 print(
1283 "The names I call myself: ",
1284 join(", ", $_[KERNEL]->alias_list()),
1285 "\n"
1286 );
1287 }
1288 );
1289 ID_id_to_session SESSION_ID
1291 ID_id_to_session() translates a session ID into a session reference.
1293 It's a special-purpose subset of alias_resolve(), so it's a little
1294 faster and somewhat less flexible.
1295 unless (defined $_[KERNEL]->ID_id_to_session($session_id)) {
1297 print "Session $session_id doesn't exist.\n";
1298 }
1299 ID_id_to_session() returns undef if a lookup failed. $! will be set to
1301 ESRCH ("No such process").
1302 ID_session_to_id SESSION_REFERENCE
1304 ID_session_to_id() converts a blessed or stringified SESSION_REFERENCE
1306 into a session ID. It's more practical for stringified references, as
1307 programs can call the POE::Session ID() method on the blessed ones.
1308 These statements are equivalent:
1309 $id = $_[SENDER]->ID();
1311 $id = $_[KERNEL]->ID_session_to_id($_[SENDER]);
1312 $id = $_[KERNEL]->ID_session_to_id("$_[SENDER]");
1313 As with other POE::Kernel lookup methods, ID_session_to_id() returns
1315 undef on failure, setting $! to ESRCH ("No such process").
1316 I/O Watchers (Selects)
1318 No event system would be complete without the ability to asynchronously
1319 watch for I/O events. POE::Kernel implements the lowest level
1320 watchers, which are called "selects" because they were historically
1321 implemented using Perl's built-in select(2) function.
1322 Applications handle I/O readiness events by performing some activity on
1324 the underlying filehandle. Read-readiness might be handled by reading
1325 from the handle. Write-readiness by writing to it.
1326 All I/O watcher events include two parameters. "ARG0" contains the
1328 handle that is ready for work. "ARG1" contains an integer describing
1329 what's ready.
1330 sub handle_io {
1332 my ($handle, $mode) = @_[ARG0, ARG1];
1333 print "File $handle is ready for ";
1334 if ($mode == 0) {
1335 print "reading";
1336 }
1337 elsif ($mode == 1) {
1338 print "writing";
1339 }
1340 elsif ($mode == 2) {
1341 print "out-of-band reading";
1342 }
1343 else {
1344 die "unknown mode $mode";
1345 }
1346 print "\n";
1347 # ... do something here
1348 }
1349 The remaining parameters, @_[ARG2..$%_], contain additional parameters
1351 that were passed to the POE::Kernel method that created the watcher.
1352 POE::Kernel conditions filehandles to be 8-bit clean and non-blocking.
1354 Programs that need them conditioned differently should set them up
1355 after starting POE I/O watchers. If you are running a Perl older than
1356 5.8.1 and is using tied filehandles, you need to set non-blocking mode
1357 yourself as IO::Handle does not work well. See
1358 <https://rt.cpan.org/Ticket/Display.html?id=67545> for more info.
1359 I/O watchers will prevent sessions from stopping.
1361 select_read FILE_HANDLE [, EVENT_NAME [, ADDITIONAL_PARAMETERS] ]
1363 select_read() starts or stops the current session from watching for
1365 incoming data on a given FILE_HANDLE. The watcher is started if
1366 EVENT_NAME is specified, or stopped if it's not.
1367 ADDITIONAL_PARAMETERS, if specified, will be passed to the EVENT_NAME
1368 handler as @_[ARG2..$#_].
1369 POE::Session->create(
1371 inline_states => {
1372 _start => sub {
1373 $_[HEAP]{socket} = IO::Socket::INET->new(
1374 PeerAddr => "localhost",
1375 PeerPort => 25,
1376 );
1377 $_[KERNEL]->select_read( $_[HEAP]{socket}, "got_input" );
1378 $_[KERNEL]->delay(timed_out => 1);
1379 },
1380 got_input => sub {
1381 my $socket = $_[ARG0];
1382 while (sysread($socket, my $buf = "", 8192)) {
1383 print $buf;
1384 }
1385 },
1386 timed_out => sub {
1387 $_[KERNEL]->select_read( delete $_[HEAP]{socket} );
1388 },
1389 }
1390 );
1391 select_read() does not return anything significant.
1393 select_write FILE_HANDLE [, EVENT_NAME [, ADDITIONAL_PARAMETERS] ]
1395 select_write() follows the same semantics as select_read(), but it
1397 starts or stops a watcher that looks for write-readiness. That is,
1398 when EVENT_NAME is delivered, it means that FILE_HANDLE is ready to be
1399 written to.
1400 select_write() does not return anything significant.
1402 select_expedite FILE_HANDLE [, EVENT_NAME [, ADDITIONAL_PARAMETERS] ]
1404 select_expedite() does the same sort of thing as select_read() and
1406 select_write(), but it watches a FILE_HANDLE for out-of-band data ready
1407 to be input from a FILE_HANDLE. Hardly anybody uses this, but it
1408 exists for completeness' sake.
1409 An EVENT_NAME event will be delivered whenever the FILE_HANDLE can be
1411 read from out-of-band. Out-of-band data is considered "expedited"
1412 because it is often ahead of a socket's normal data.
1413 select_expedite() does not return anything significant.
1415 select_pause_read FILE_HANDLE
1417 select_pause_read() is a lightweight way to pause a FILE_HANDLE input
1419 watcher without performing all the bookkeeping of a select_read().
1420 It's used with select_resume_read() to implement input flow control.
1421 Input that occurs on FILE_HANDLE will backlog in the operating system
1423 buffers until select_resume_read() is called.
1424 A side effect of bypassing the select_read() bookkeeping is that a
1426 paused FILE_HANDLE will not prematurely stop the current session.
1427 select_pause_read() does not return anything significant.
1429 select_resume_read FILE_HANDLE
1431 select_resume_read() resumes a FILE_HANDLE input watcher that was
1433 previously paused by select_pause_read(). See select_pause_read() for
1434 more discussion on lightweight input flow control.
1435 Data backlogged in the operating system due to a select_pause_read()
1437 call will become available after select_resume_read() is called.
1438 select_resume_read() does not return anything significant.
1440 select_pause_write FILE_HANDLE
1442 select_pause_write() pauses a FILE_HANDLE output watcher the same way
1444 select_pause_read() does for input. Please see select_pause_read() for
1445 further discussion.
1446 select_resume_write FILE_HANDLE
1448 select_resume_write() resumes a FILE_HANDLE output watcher the same way
1450 that select_resume_read() does for input. See select_resume_read() for
1451 further discussion.
1452 select FILE_HANDLE [, EV_READ [, EV_WRITE [, EV_EXPEDITE [, ARGS] ] ] ]
1454 POE::Kernel's select() method sets or clears a FILE_HANDLE's read,
1456 write and expedite watchers at once. It's a little more expensive than
1457 calling select_read(), select_write() and select_expedite() manually,
1458 but it's significantly more convenient.
1459 Defined event names enable their corresponding watchers, and undefined
1461 event names disable them. This turns off all the watchers for a
1462 FILE_HANDLE:
1463 sub stop_io {
1465 $_[KERNEL]->select( $_[HEAP]{file_handle} );
1466 }
1467 This statement:
1469 $_[KERNEL]->select( $file_handle, undef, "write_event", undef, @stuff );
1471 is equivalent to:
1473 $_[KERNEL]->select_read( $file_handle );
1475 $_[KERNEL]->select_write( $file_handle, "write_event", @stuff );
1476 $_[KERNEL]->select_expedite( $file_handle );
1477 POE::Kernel's select() should not be confused with Perl's built-in
1479 select() function.
1480 As with the other I/O watcher methods, select() does not return a
1482 meaningful value.
1483 Session Management
1485 Sessions are dynamic. They may be created and destroyed during a
1486 program's lifespan. When a session is created, it becomes the "child"
1487 of the current session. The creator -- the current session -- becomes
1488 its "parent" session. This is loosely modeled after UNIX processes.
1489 The most common session management is done by creating new sessions and
1491 allowing them to eventually stop.
1492 Every session has a parent, even the very first session created.
1494 Sessions without obvious parents are children of the program's
1495 POE::Kernel instance.
1496 Child sessions will keep their parents active. See "Session Lifespans"
1498 for more about why sessions stay alive.
1499 The parent/child relationship tree also governs the way many signals
1501 are dispatched. See "Common Signal Dispatching" for more information
1502 on that.
1503 Session Management Events (_start, _stop, _parent, _child)
1505 POE::Kernel provides four session management events: _start, _stop,
1507 _parent and _child. They are invoked synchronously whenever a session
1508 is newly created or just about to be destroyed.
1509 _start
1511 _start should be familiar by now. POE dispatches the _start event to
1512 initialize a session after it has been registered under POE::Kernel.
1513 What is not readily apparent, however, is that it is invoked before
1514 the POE::Session constructor returns.
1515 Within the _start handler, the event's sender is the session that
1517 created the new session. Otherwise known as the new session's
1518 parent. Sessions created before POE::Kernel->run() is called will be
1519 descendents of the program's POE::Kernel singleton.
1520 The _start handler's return value is passed to the parent session in
1522 a _child event, along with the notification that the parent's new
1523 child was created successfully. See the discussion of _child for
1524 more details.
1525 POE::Session->create(
1527 inline_states => { _start=> \&_start },
1528 args => [ $some, $args ]
1529 );
1530 sub _start {
1532 my ( $some, $args ) = @_[ ARG0, ARG1 ];
1533 # ....
1534 }
1535 _stop
1537 _stop is a little more mysterious. POE calls a _stop handler when a
1538 session is irrevocably about to be destroyed. Part of session
1539 destruction is the forcible reclamation of its resources (events,
1540 timers, message events, etc.) so it's not possible to post() a
1541 message from _stop's handler. A program is free to try, but the
1542 event will be destroyed before it has a chance to be dispatched.
1543 the _stop handler's return value is passed to the parent's _child
1545 event. See _child for more details.
1546 _stop is usually invoked when a session has no further reason to
1548 live, although signals may cause them to stop sooner.
1549 The corresponding _child handler is invoked synchronously just after
1551 _stop returns.
1552 _parent
1554 _parent is used to notify a child session when its parent has
1555 changed. This usually happens when a session is first created. It
1556 can also happen when a child session is detached from its parent. See
1557 detach_child and "detach_myself".
1558 _parent's ARG0 contains the session's previous parent, and ARG1
1560 contains its new parent.
1561 sub _parent {
1563 my ( $old_parent, $new_parent ) = @_[ ARG0, ARG1 ];
1564 print(
1565 "Session ", $_[SESSION]->ID,
1566 " parent changed from session ", $old_parent->ID,
1567 " to session ", $new_parent->ID,
1568 "\n"
1569 );
1570 }
1571 _child
1573 _child notifies one session when a child session has been created,
1574 destroyed, or reassigned to or from another parent. It's usually
1575 dispatched when sessions are created or destroyed. It can also
1576 happen when a session is detached from its parent.
1577 _child includes some information in the "arguments" portion of @_.
1579 Typically ARG0, ARG1 and ARG2, but these may be overridden by a
1580 different POE::Session class:
1581 ARG0 contains a string describing what has happened to the child.
1583 The string may be 'create' (the child session has been created),
1584 'gain' (the child has been given by another session), or 'lose' (the
1585 child session has stopped or been given away).
1586 In all cases, ARG1 contains a reference to the child session.
1588 In the 'create' case, ARG2 holds the value returned by the child
1590 session's _start handler. Likewise, ARG2 holds the _stop handler's
1591 return value for the 'lose' case.
1592 sub _child {
1594 my( $reason, $child ) = @_[ ARG0, ARG1 ];
1595 if( $reason eq 'create' ) {
1596 my $retval = $_[ ARG2 ];
1597 }
1598 # ...
1599 }
1600 The events are delivered in specific orders.
1602 When a new session is created:
1604 1. The session's constructor is called.
1606 2. The session is put into play. That is, POE::Kernel enters the
1608 session into its bookkeeping.
1609 3. The new session receives _start.
1611 4. The parent session receives _child ('create'), the new session
1613 reference, and the new session's _start's return value.
1614 5. The session's constructor returns.
1616 When an old session stops:
1618 1. If the session has children of its own, they are given to the
1620 session's parent. This triggers one or more _child ('gain') events
1621 in the parent, and a _parent in each child.
1622 2. Once divested of its children, the stopping session receives a
1624 _stop event.
1625 3. The stopped session's parent receives a _child ('lose') event with
1627 the departing child's reference and _stop handler's return value.
1628 4. The stopped session is removed from play, as are all its remaining
1630 resources.
1631 5. The parent session is checked for idleness. If so, garbage
1633 collection will commence on it, and it too will be stopped
1634 When a session is detached from its parent:
1636 1. The parent session of the session being detached is notified with a
1638 _child ('lose') event. The _stop handler's return value is undef
1639 since the child is not actually stopping.
1640 2. The detached session is notified with a _parent event that its new
1642 parent is POE::Kernel itself.
1643 3. POE::Kernel's bookkeeping data is adjusted to reflect the change of
1645 parentage.
1646 4. The old parent session is checked for idleness. If so, garbage
1648 collection will commence on it, and it too will be stopped
1649 Session Management Methods
1651 These methods allow sessions to be detached from their parents in the
1653 rare cases where the parent/child relationship gets in the way.
1654 detach_child CHILD_SESSION
1656 detach_child() detaches a particular CHILD_SESSION from the current
1658 session. On success, the CHILD_SESSION will become a child of the
1659 POE::Kernel instance, and detach_child() will return true. On failure
1660 however, detach_child() returns false and sets $! to explain the nature
1661 of the failure:
1662 ESRCH ("No such process").
1664 The CHILD_SESSION is not a valid session.
1665 EPERM ("Operation not permitted").
1667 The CHILD_SESSION exists, but it is not a child of the current
1668 session.
1669 detach_child() will generate "_parent" and/or "_child" events to the
1671 appropriate sessions. See Session Management Events for a detailed
1672 explanation of these events. See above for the order the events are
1673 generated.
1674 detach_myself
1676 detach_myself() detaches the current session from its current parent.
1678 The new parent will be the running POE::Kernel instance. It returns
1679 true on success. On failure it returns false and sets $! to explain
1680 the nature of the failure:
1681 EPERM ("Operation not permitted").
1683 The current session is already a child of POE::Kernel, so it may
1684 not be detached.
1685 detach_child() will generate "_parent" and/or "_child" events to the
1687 appropriate sessions. See Session Management Events for a detailed
1688 explanation of these events. See above for the order the events are
1689 generated.
1690 Signals
1692 POE::Kernel provides methods through which a program can register
1693 interest in signals that come along, can deliver its own signals
1694 without resorting to system calls, and can indicate that signals have
1695 been handled so that default behaviors are not necessary.
1696 Signals are action at a distance by nature, and their implementation
1698 requires widespread synchronization between sessions (and reentrancy in
1699 the dispatcher, but that's an implementation detail). Perfecting the
1700 semantics has proven difficult, but POE tries to do the Right Thing
1701 whenever possible.
1702 POE does not register %SIG handlers for signals until sig() is called
1704 to watch for them. Therefore a signal's default behavior occurs for
1705 unhandled signals. That is, SIGINT will gracelessly stop a program,
1706 SIGWINCH will do nothing, SIGTSTP will pause a program, and so on.
1707 Signal Classes
1709 There are three signal classes. Each class defines a default behavior
1711 for the signal and whether the default can be overridden. They are:
1712 Benign, advisory, or informative signals
1714 These are three names for the same signal class. Signals in this class
1716 notify a session of an event but do not terminate the session if they
1717 are not handled.
1718 It is possible for an application to create its own benign signals.
1720 See "signal" below.
1721 Terminal signals
1723 Terminal signals will kill sessions if they are not handled by a
1725 "sig_handled"() call. The OS signals that usually kill or dump a
1726 process are considered terminal in POE, but they never trigger a
1727 coredump. These are: HUP, INT, QUIT and TERM.
1728 There are two terminal signals created by and used within POE:
1730 DIE "DIE" notifies sessions that a Perl exception has occurred. See
1732 "Exception Handling" for details.
1733 IDLE
1735 The "IDLE" signal is used to notify leftover sessions that a
1736 program has run out of things to do.
1737 Nonmaskable signals
1739 Nonmaskable signals are terminal regardless whether sig_handled() is
1741 called. The term comes from "NMI", the non-maskable CPU interrupt
1742 usually generated by an unrecoverable hardware exception.
1743 Sessions that receive a non-maskable signal will unavoidably stop. POE
1745 implements two non-maskable signals:
1746 ZOMBIE
1748 This non-maskable signal is fired if a program has received an
1749 "IDLE" signal but neither restarted nor exited. The program has
1750 become a zombie (that is, it's neither dead nor alive, and only
1751 exists to consume braaaains ...er... memory). The "ZOMBIE" signal
1752 acts like a cricket bat to the head, bringing the zombie down, for
1753 good.
1754 UIDESTROY
1756 This non-maskable signal indicates that a program's user interface
1757 has been closed, and the program should take the user's hint and
1758 buzz off as well. It's usually generated when a particular GUI
1759 widget is closed.
1760 Common Signal Dispatching
1762 Most signals are not dispatched to a single session. POE's session
1764 lineage (parents and children) form a sort of family tree. When a
1765 signal is sent to a session, it first passes through any children (and
1766 grandchildren, and so on) that are also interested in the signal.
1767 In the case of terminal signals, if any of the sessions a signal passes
1769 through calls "sig_handled"(), then the signal is considered taken care
1770 of. However if none of them do, then the entire session tree rooted at
1771 the destination session is terminated. For example, consider this tree
1772 of sessions:
1773 POE::Kernel
1775 Session 2
1776 Session 4
1777 Session 5
1778 Session 3
1779 Session 6
1780 Session 7
1781 POE::Kernel is the parent of sessions 2 and 3. Session 2 is the parent
1783 of sessions 4 and 5. And session 3 is the parent of 6 and 7.
1784 A signal sent to Session 2 may also be dispatched to session 4 and 5
1786 because they are 2's children. Sessions 4 and 5 will only receive the
1787 signal if they have registered the appropriate watcher. If the signal
1788 is terminal, and none of the signal watchers in sessions 2, 4 and 5
1789 called "sig_handled()", all 3 sessions will be terminated.
1790 The program's POE::Kernel instance is considered to be a session for
1792 the purpose of signal dispatch. So any signal sent to POE::Kernel will
1793 propagate through every interested session in the entire program. This
1794 is in fact how OS signals are handled: A global signal handler is
1795 registered to forward the signal to POE::Kernel.
1796 Signal Semantics
1798 All signals come with the signal name in ARG0. The signal name is as
1800 it appears in %SIG, with one exception: Child process signals are
1801 always "CHLD" even if the current operating system recognizes them as
1802 "CLD".
1803 Certain signals have special semantics:
1805 SIGCHLD
1807 SIGCLD
1809 Both "SIGCHLD" and "SIGCLD" indicate that a child process has exited or
1811 been terminated by some signal. The actual signal name varies between
1812 operating systems, but POE uses "CHLD" regardless.
1813 Interest in "SIGCHLD" is registered using the "sig_child" method. The
1815 "sig"() method also works, but it's not as nice.
1816 The "SIGCHLD" event includes three parameters:
1818 ARG0
1820 "ARG0" contains the string 'CHLD' (even if the OS calls it SIGCLD,
1821 SIGMONKEY, or something else).
1822 ARG1
1824 "ARG1" contains the process ID of the finished child process.
1825 ARG2
1827 And "ARG2" holds the value of $? for the finished process.
1828 Example:
1830 sub sig_CHLD {
1832 my( $name, $PID, $exit_val ) = @_[ ARG0, ARG1, ARG2 ];
1833 # ...
1834 }
1835 SIGPIPE
1837 SIGPIPE is rarely used since POE provides events that do the same
1839 thing. Nevertheless SIGPIPE is supported if you need it. Unlike most
1840 events, however, SIGPIPE is dispatched directly to the active session
1841 when it's caught. Barring race conditions, the active session should
1842 be the one that caused the OS to send the signal in the first place.
1843 The SIGPIPE signal will still propagate to child sessions.
1845 ARG0 is "PIPE". There is no other information associated with this
1847 signal.
1848 SIGWINCH
1850 Window resizes can generate a large number of signals very quickly.
1852 This may not be a problem when using perl 5.8.0 or later, but earlier
1853 versions may not take kindly to such abuse. You have been warned.
1854 ARG0 is "WINCH". There is no other information associated with this
1856 signal.
1857 Exception Handling
1859 POE::Kernel provides only one form of exception handling: the "DIE"
1861 signal.
1862 When exception handling is enabled (the default), POE::Kernel wraps
1864 state invocation in "eval{}". If the event handler raises an
1865 exception, generally with "die", POE::Kernel will dispatch a "DIE"
1866 signal to the event's destination session.
1867 "ARG0" is the signal name, "DIE".
1869 "ARG1" is a hashref describing the exception:
1871 error_str
1873 The text of the exception. In other words, $@.
1874 dest_session
1876 Session object of the state that the raised the exception. In
1877 other words, $_[SESSION] in the function that died.
1878 event
1880 Name of the event that died.
1881 source_session
1883 Session object that sent the original event. That is, $_[SENDER]
1884 in the function that died.
1885 from_state
1887 State from which the original event was sent. That is,
1888 $_[CALLER_STATE] in the function that died.
1889 file
1891 Name of the file the event was sent from. That is, $_[CALLER_FILE]
1892 in the function that died.
1893 line
1895 Line number the event was sent from. That is, $_[CALLER_LINE] in
1896 the function that died.
1897 Note that the preceding discussion assumes you are using POE::Session's
1899 call semantics.
1900 Note that the "DIE" signal is sent to the session that raised the
1902 exception, not the session that sent the event that caused the
1903 exception to be raised.
1904 sub _start {
1906 $poe_kernel->sig( DIE => 'sig_DIE' );
1907 $poe_kernel->yield( 'some_event' );
1908 }
1909 sub some_event {
1911 die "I didn't like that!";
1912 }
1913 sub sig_DIE {
1915 my( $sig, $ex ) = @_[ ARG0, ARG1 ];
1916 # $sig is 'DIE'
1917 # $ex is the exception hash
1918 warn "$$: error in $ex->{event}: $ex->{error_str}";
1919 $poe_kernel->sig_handled();
1920 # Send the signal to session that sent the original event.
1922 if( $ex->{source_session} ne $_[SESSION] ) {
1923 $poe_kernel->signal( $ex->{source_session}, 'DIE', $sig, $ex );
1924 }
1925 }
1926 POE::Kernel's built-in exception handling can be disabled by setting
1928 the "POE::Kernel::CATCH_EXCEPTIONS" constant to zero. As with other
1929 compile-time configuration constants, it must be set before POE::Kernel
1930 is compiled:
1931 BEGIN {
1933 package POE::Kernel;
1934 use constant CATCH_EXCEPTIONS => 0;
1935 }
1936 use POE;
1937 or
1939 sub POE::Kernel::CATCH_EXCEPTIONS () { 0 }
1941 use POE;
1942 Signal Watcher Methods
1944 And finally the methods themselves.
1945 sig SIGNAL_NAME [, EVENT_NAME [, LIST] ]
1947 sig() registers or unregisters an EVENT_NAME event for a particular
1949 SIGNAL_NAME, with an optional LIST of parameters that will be passed to
1950 the signal's handler---after any data that comes wit the signal.
1951 If EVENT_NAME is defined, the signal handler is registered. Otherwise
1953 it's unregistered.
1954 Each session can register only one handler per SIGNAL_NAME. Subsequent
1956 registrations will replace previous ones. Multiple sessions may
1957 however watch the same signal.
1958 SIGNAL_NAMEs are generally the same as members of %SIG, with two
1960 exceptions. First, "CLD" is an alias for "CHLD" (although see
1961 "sig_child"). And second, it's possible to send and handle signals
1962 created by the application and have no basis in the operating system.
1963 sub handle_start {
1965 $_[KERNEL]->sig( INT => "event_ui_shutdown" );
1966 $_[KERNEL]->sig( bat => "holy_searchlight_batman" );
1967 $_[KERNEL]->sig( signal => "main_screen_turn_on" );
1968 }
1969 The operating system may never be able to generate the last two
1971 signals, but a POE session can by using POE::Kernel's "signal"()
1972 method.
1973 Later on the session may decide not to handle the signals:
1975 sub handle_ui_shutdown {
1977 $_[KERNEL]->sig( "INT" );
1978 $_[KERNEL]->sig( "bat" );
1979 $_[KERNEL]->sig( "signal" );
1980 }
1981 More than one session may register interest in the same signal, and a
1983 session may clear its own signal watchers without affecting those in
1984 other sessions.
1985 sig() does not return a meaningful value.
1987 sig_child PROCESS_ID [, EVENT_NAME [, LIST] ]
1989 sig_child() is a convenient way to deliver an EVENT_NAME event when a
1991 particular PROCESS_ID has exited. An optional LIST of parameters will
1992 be passed to the signal handler after the waitpid() information.
1993 The watcher can be cleared at any time by calling sig_child() with just
1995 the PROCESS_ID.
1996 A session may register as many sig_child() handlers as necessary, but a
1998 session may only have one per PROCESS_ID.
1999 sig_child() watchers are one-shot. They automatically unregister
2001 themselves once the EVENT_NAME has been delivered. There's no point in
2002 continuing to watch for a signal that will never come again. Other
2003 signal handlers persist until they are cleared.
2004 sig_child() watchers keep a session alive for as long as they are
2006 active. This is unique among POE's signal watchers.
2007 Programs that wish to reliably reap child processes should be sure to
2009 call sig_child() before returning from the event handler that forked
2010 the process. Otherwise POE::Kernel may have an opportunity to call
2011 waitpid() before an appropriate event watcher has been registered.
2012 Programs that reap processes with waitpid() must clear POE's watchers
2014 for the same process IDs, otherwise POE will wait indefinitely for
2015 processes that never send signals.
2016 sig_child() does not return a meaningful value.
2018 sub forked_parent {
2020 my( $heap, $pid, $details ) = @_[ HEAP, ARG0, ARG1 ];
2021 $poe_kernel->sig_child( $pid, 'sig_child', $details );
2022 }
2023 sub sig_child {
2025 my( $heap, $sig, $pid, $exit_val, $details ) = @_[ HEAP, ARG0..ARG3 ];
2026 my $details = delete $heap->{ $pid };
2027 warn "$$: Child $pid exited"
2028 # .... also, $details has been passed from forked_parent()
2029 # through sig_child()
2030 }
2031 sig_handled
2033 sig_handled() informs POE::Kernel that the currently dispatched signal
2035 has been handled by the currently active session. If the signal is
2036 terminal, the sig_handled() call prevents POE::Kernel from stopping the
2037 sessions that received the signal.
2038 A single signal may be dispatched to several sessions. Only one needs
2040 to call sig_handled() to prevent the entire group from being stopped.
2041 If none of them call it, however, then they are all stopped together.
2042 sig_handled() does not return a meaningful value.
2044 sub _start {
2046 $_[KERNEL]->sig( INT => 'sig_INT' );
2047 }
2048 sub sig_INT {
2050 warn "$$ SIGINT";
2051 $_[KERNEL]->sig_handled();
2052 }
2053 signal SESSION, SIGNAL_NAME [, ARGS_LIST]
2055 signal() posts a SIGNAL_NAME signal to a specific SESSION with an
2057 optional ARGS_LIST that will be passed to every interested handler. As
2058 mentioned elsewhere, the signal may be delivered to SESSION's children,
2059 grandchildren, and so on. And if SESSION is the POE::Kernel itself,
2060 then all interested sessions will receive the signal.
2061 It is possible to send a signal in POE that doesn't exist in the
2063 operating system. signal() places the signal directly into POE's event
2064 queue as if they came from the operating system, but they are not
2065 limited to signals recognized by kill(). POE uses a few of these
2066 fictitious signals for its own global notifications.
2067 For example:
2069 sub some_event_handler {
2071 # Turn on all main screens.
2072 $_[KERNEL]->signal( $_[KERNEL], "signal" );
2073 }
2074 signal() returns true on success. On failure, it returns false after
2076 setting $! to explain the nature of the failure:
2077 ESRCH ("No such process")
2079 The SESSION does not exist.
2080 Because all sessions are a child of POE::Kernel, sending a signal to
2082 the kernel will propagate the signal to all sessions. This is a cheap
2083 form of multicast.
2084 $_[KERNEL]->signal( $_[KERNEL], 'shutdown' );
2086 signal_ui_destroy WIDGET_OBJECT
2088 signal_ui_destroy() associates the destruction of a particular
2090 WIDGET_OBJECT with the complete destruction of the program's user
2091 interface. When the WIDGET_OBJECT destructs, POE::Kernel issues the
2092 non-maskable UIDESTROY signal, which quickly triggers mass destruction
2093 of all active sessions. POE::Kernel->run() returns shortly thereafter.
2094 sub setup_ui {
2096 $_[HEAP]{main_widget} = Gtk->new("toplevel");
2097 # ... populate the main widget here ...
2098 $_[KERNEL]->signal_ui_destroy( $_[HEAP]{main_widget} );
2099 }
2100 Detecting widget destruction is specific to each toolkit.
2102 Event Handler Management
2104 Event handler management methods let sessions hot swap their event
2105 handlers at run time. For example, the POE::Wheel objects use state()
2106 to dynamically mix their own event handlers into the sessions that
2107 create them.
2108 These methods only affect the current session; it would be rude to
2110 change another session's handlers.
2111 There is only one method in this group. Since it may be called in
2113 several different ways, it may be easier to understand if each is
2114 documented separately.
2115 state EVENT_NAME [, CODE_REFERNCE]
2117 state() sets or removes a handler for EVENT_NAME in the current
2119 session. The function referred to by CODE_REFERENCE will be called
2120 whenever EVENT_NAME events are dispatched to the current session. If
2121 CODE_REFERENCE is omitted, the handler for EVENT_NAME will be removed.
2122 A session may only have one handler for a given EVENT_NAME. Subsequent
2124 attempts to set an EVENT_NAME handler will replace earlier handlers
2125 with the same name.
2126 # Stop paying attention to input. Say goodbye, and
2128 # trigger a socket close when the message is sent.
2129 sub send_final_response {
2130 $_[HEAP]{wheel}->put("KTHXBYE");
2131 $_[KERNEL]->state( 'on_client_input' );
2132 $_[KERNEL]->state( on_flush => \&close_connection );
2133 }
2134 state EVENT_NAME [, OBJECT_REFERENCE [, OBJECT_METHOD_NAME] ]
2136 Set or remove a handler for EVENT_NAME in the current session. If an
2138 OBJECT_REFERENCE is given, that object will handle the event. An
2139 optional OBJECT_METHOD_NAME may be provided. If the method name is not
2140 given, POE will look for a method matching the EVENT_NAME instead. If
2141 the OBJECT_REFERENCE is omitted, the handler for EVENT_NAME will be
2142 removed.
2143 A session may only have one handler for a given EVENT_NAME. Subsequent
2145 attempts to set an EVENT_NAME handler will replace earlier handlers
2146 with the same name.
2147 $_[KERNEL]->state( 'some_event', $self );
2149 $_[KERNEL]->state( 'other_event', $self, 'other_method' );
2150 state EVENT_NAME [, CLASS_NAME [, CLASS_METHOD_NAME] ]
2152 This form of state() call is virtually identical to that of the object
2154 form.
2155 Set or remove a handler for EVENT_NAME in the current session. If an
2157 CLASS_NAME is given, that class will handle the event. An optional
2158 CLASS_METHOD_NAME may be provided. If the method name is not given,
2159 POE will look for a method matching the EVENT_NAME instead. If the
2160 CLASS_NAME is omitted, the handler for EVENT_NAME will be removed.
2161 A session may only have one handler for a given EVENT_NAME. Subsequent
2163 attempts to set an EVENT_NAME handler will replace earlier handlers
2164 with the same name.
2165 $_[KERNEL]->state( 'some_event', __PACKAGE__ );
2167 $_[KERNEL]->state( 'other_event', __PACKAGE__, 'other_method' );
2168 Public Reference Counters
2170 The methods in this section manipulate reference counters on the
2171 current session or another session.
2172 Each session has a namespace for user-manipulated reference counters.
2174 These namespaces are associated with the target SESSION_ID for the
2175 reference counter methods, not the caller. Nothing currently prevents
2176 one session from decrementing a reference counter that was incremented
2177 by another, but this behavior is not guaranteed to remain. For now,
2178 it's up to the users of these methods to choose obscure counter names
2179 to avoid conflicts.
2180 Reference counting is a big part of POE's magic. Various objects
2182 (mainly event watchers and components) hold references to the sessions
2183 that own them. "Session Lifespans" explains the concept in more
2184 detail.
2185 The ability to keep a session alive is sometimes useful in an
2187 application or library. For example, a component may hold a public
2188 reference to another session while it processes a request from that
2189 session. In doing so, the component guarantees that the requester is
2190 still around when a response is eventually ready. Keeping a reference
2191 to the session's object is not enough. POE::Kernel has its own
2192 internal reference counting mechanism.
2193 refcount_increment SESSION_ID, COUNTER_NAME
2195 refcount_increment() increases the value of the COUNTER_NAME reference
2197 counter for the session identified by a SESSION_ID. To discourage the
2198 use of session references, the refcount_increment() target session must
2199 be specified by its session ID.
2200 The target session will not stop until the value of any and all of its
2202 COUNTER_NAME reference counters are zero. (Actually, it may stop in
2203 some cases, such as failing to handle a terminal signal.)
2204 Negative reference counters are legal. They still must be incremented
2206 back to zero before a session is eligible for stopping.
2207 sub handle_request {
2209 # Among other things, hold a reference count on the sender.
2210 $_[KERNEL]->refcount_increment( $_[SENDER]->ID, "pending request");
2211 $_[HEAP]{requesters}{$request_id} = $_[SENDER]->ID;
2212 }
2213 For this to work, the session needs a way to remember the
2215 $_[SENDER]->ID for a given request. Customarily the session generates
2216 a request ID and uses that to track the request until it is fulfilled.
2217 refcount_increment() returns the resulting reference count (which may
2219 be zero) on success. On failure, it returns undef and sets $! to be
2220 the reason for the error.
2221 ESRCH: The SESSION_ID does not refer to a currently active session.
2223 refcount_decrement SESSION_ID, COUNTER_NAME
2225 refcount_decrement() reduces the value of the COUNTER_NAME reference
2227 counter for the session identified by a SESSION_ID. It is the
2228 counterpoint for refcount_increment(). Please see refcount_increment()
2229 for more context.
2230 sub finally_send_response {
2232 # Among other things, release the reference count for the
2233 # requester.
2234 my $requester_id = delete $_[HEAP]{requesters}{$request_id};
2235 $_[KERNEL]->refcount_decrement( $requester_id, "pending request");
2236 }
2237 The requester's $_[SENDER]->ID is remembered and removed from the heap
2239 (lest there be memory leaks). It's used to decrement the reference
2240 counter that was incremented at the start of the request.
2241 refcount_decrement() returns the resulting reference count (which may
2243 be zero) on success. On failure, it returns undef, and $! will be set
2244 to the reason for the failure:
2245 ESRCH: The SESSION_ID does not refer to a currently active session.
2247 It is not possible to discover currently active public references. See
2249 POE::API::Peek.
2250 Kernel State Accessors
2252 POE::Kernel provides a few accessors into its massive brain so that
2253 library developers may have convenient access to necessary data without
2254 relying on their callers to provide it.
2255 These accessors expose ways to break session encapsulation. Please use
2257 them sparingly and carefully.
2258 get_active_session
2260 get_active_session() returns a reference to the session that is
2262 currently running, or a reference to the program's POE::Kernel instance
2263 if no session is running at that moment. The value is equivalent to
2264 POE::Session's $_[SESSION].
2265 This method was added for libraries that need $_[SESSION] but don't
2267 want to include it as a parameter in their APIs.
2268 sub some_housekeeping {
2270 my( $self ) = @_;
2271 my $session = $poe_kernel->get_active_session;
2272 # do some housekeeping on $session
2273 }
2274 get_active_event
2276 get_active_event() returns the name of the event currently being
2278 dispatched. It returns an empty string when called outside event
2279 dispatch. The value is equivalent to POE::Session's $_[STATE].
2280 sub waypoint {
2282 my( $message ) = @_;
2283 my $event = $poe_kernel->get_active_event;
2284 print STDERR "$$:$event:$mesage\n";
2285 }
2286 get_event_count
2288 get_event_count() returns the number of events pending in POE's event
2290 queue. It is exposed for POE::Loop class authors. It may be
2291 deprecated in the future.
2292 get_next_event_time
2294 get_next_event_time() returns the time the next event is due, in a form
2296 compatible with the UNIX time() function. It is exposed for POE::Loop
2297 class authors. It may be deprecated in the future.
2298 poe_kernel_loop
2300 poe_kernel_loop() returns the name of the POE::Loop class that is used
2302 to detect and dispatch events.
2303 Session Helper Methods
2305 The methods in this group expose features for POE::Session class
2306 authors.
2307 session_alloc SESSION_OBJECT [, START_ARGS]
2309 session_alloc() allocates a session context within POE::Kernel for a
2311 newly created SESSION_OBJECT. A list of optional START_ARGS will be
2312 passed to the session as part of the "_start" event.
2313 The SESSION_OBJECT is expected to follow a subset of POE::Session's
2315 interface.
2316 There is no session_free(). POE::Kernel determines when the session
2318 should stop and performs the necessary cleanup after dispatching _stop
2319 to the session.
2320 Miscellaneous Methods
2322 We don't know where to classify the methods in this section.
2323 new
2325 It is not necessary to call POE::Kernel's new() method. Doing so will
2327 return the program's singleton POE::Kernel object, however.
2328
2330 POE::Kernel exports two variables for your coding enjoyment:
2331 $poe_kernel and $poe_main_window. POE::Kernel is implicitly used by
2332 POE itself, so using POE gets you POE::Kernel (and its exports) for
2333 free.
2334 In more detail:
2336 $poe_kernel
2338 $poe_kernel contains a reference to the process' POE::Kernel singleton
2339 instance. It's mainly used for accessing POE::Kernel methods from
2340 places where $_[KERNEL] is not available. It's most commonly used in
2341 helper libraries.
2342 $poe_main_window
2344 $poe_main_window is used by graphical toolkits that require at least
2345 one widget to be created before their event loops are usable. This is
2346 currently only Tk.
2347 POE::Loop::Tk creates a main window to satisfy Tk's event loop. The
2349 window is given to the application since POE has no other use for it.
2350 $poe_main_window is undefined in toolkits that don't require a widget
2352 to dispatch events.
2353 On a related note, POE will shut down if the widget in $poe_main_window
2355 is destroyed. This can be changed with POE::Kernel's
2356 "signal_ui_destroy" method.
2357
2359 POE includes quite a lot of debugging code, in the form of both fatal
2360 assertions and run-time traces. They may be enabled at compile time,
2361 but there is no way to toggle them at run-time. This was done to avoid
2362 run-time penalties in programs where debugging is not necessary. That
2363 is, in most production cases.
2364 Traces are verbose reminders of what's going on within POE. Each is
2366 prefixed with a four-character field describing the POE subsystem that
2367 generated it.
2368 Assertions (asserts) are quiet but deadly, both in performance (they
2370 cause a significant run-time performance hit) and because they cause
2371 fatal errors when triggered.
2372 The assertions and traces are useful for developing programs with POE,
2374 but they were originally added to debug POE itself.
2375 Each assertion and tracing group is enabled by setting a constant in
2377 the POE::Kernel namespace to a true value.
2378 BEGIN {
2380 package POE::Kernel;
2381 use constant ASSERT_DEFAULT => 1;
2382 }
2383 use POE;
2384 Or the old-fashioned (and more concise) "constant subroutine" method.
2386 This doesn't need the "BEGIN{}" block since subroutine definitions are
2387 done at compile time.
2388 sub POE::Kernel::ASSERT_DEFAULT () { 1 }
2390 use POE;
2391 The switches must be defined as constants before POE::Kernel is first
2393 loaded. Otherwise Perl's compiler will not see the constants when
2394 first compiling POE::Kernel, and the features will not be properly
2395 enabled.
2396 Assertions and traces may also be enabled by setting shell environment
2398 variables. The environment variables are named after the POE::Kernel
2399 constants with a "POE_" prefix.
2400 POE_ASSERT_DEFAULT=1 POE_TRACE_DEFAULT=1 ./my_poe_program
2402 In alphabetical order:
2404 ASSERT_DATA
2406 ASSERT_DATA enables run-time data integrity checks within POE::Kernel
2407 and the classes that mix into it. POE::Kernel tracks a lot of cross-
2408 referenced data, and this group of assertions ensures that it's
2409 consistent.
2410 Prefix: <dt>
2412 Environment variable: POE_ASSERT_DATA
2414 ASSERT_DEFAULT
2416 ASSERT_DEFAULT specifies the default value for assertions that are not
2417 explicitly enabled or disabled. This is a quick and reliable way to
2418 make sure all assertions are on.
2419 No assertion uses ASSERT_DEFAULT directly, and this assertion flag has
2421 no corresponding output prefix.
2422 Turn on all assertions except ASSERT_EVENTS:
2424 sub POE::Kernel::ASSERT_DEFAULT () { 1 }
2426 sub POE::Kernel::ASSERT_EVENTS () { 0 }
2427 use POE::Kernel;
2428 Prefix: (none)
2430 Environment variable: POE_ASSERT_DEFAULT
2432 ASSERT_EVENTS
2434 ASSERT_EVENTS mainly checks for attempts to dispatch events to sessions
2435 that don't exist. This assertion can assist in the debugging of
2436 strange, silent cases where event handlers are not called.
2437 Prefix: <ev>
2439 Environment variable: POE_ASSERT_EVENTS
2441 ASSERT_FILES
2443 ASSERT_FILES enables some run-time checks in POE's filehandle watchers
2444 and the code that manages them.
2445 Prefix: <fh>
2447 Environment variable: POE_ASSERT_FILES
2449 ASSERT_RETVALS
2451 ASSERT_RETVALS upgrades failure codes from POE::Kernel's methods from
2452 advisory return values to fatal errors. Most programmers don't check
2453 the values these methods return, so ASSERT_RETVALS is a quick way to
2454 validate one's assumption that all is correct.
2455 Prefix: <rv>
2457 Environment variable: POE_ASSERT_RETVALS
2459 ASSERT_USAGE
2461 ASSERT_USAGE is the counterpoint to ASSERT_RETVALS. It enables run-
2462 time checks that the parameters to POE::Kernel's methods are correct.
2463 It's a quick (but not foolproof) way to verify a program's use of POE.
2464 Prefix: <us>
2466 Environment variable: POE_ASSERT_USAGE
2468 TRACE_DEFAULT
2470 TRACE_DEFAULT specifies the default value for traces that are not
2471 explicitly enabled or disabled. This is a quick and reliable way to
2472 ensure your program generates copious output on the file named in
2473 TRACE_FILENAME or STDERR by default.
2474 To enable all traces except a few noisier ones:
2476 sub POE::Kernel::TRACE_DEFAULT () { 1 }
2478 sub POE::Kernel::TRACE_EVENTS () { 0 }
2479 use POE::Kernel;
2480 Prefix: (none)
2482 Environment variable: POE_TRACE_DEFAULT
2484 TRACE_DESTROY
2486 TRACE_DESTROY causes every POE::Session object to dump the contents of
2487 its $_[HEAP] when Perl destroys it. This trace was added to help
2488 developers find memory leaks in their programs.
2489 Prefix: A line that reads "----- Session $self Leak Check -----".
2491 Environment variable: POE_TRACE_DESTROY
2493 TRACE_EVENTS
2495 TRACE_EVENTS enables messages pertaining to POE's event queue's
2496 activities: when events are enqueued, dispatched or discarded, and
2497 more. It's great for determining where events go and when.
2498 Understandably this is one of POE's more verbose traces.
2499 Prefix: <ev>
2501 Environment variable: POE_TRACE_EVENTS
2503 TRACE_FILENAME
2505 TRACE_FILENAME specifies the name of a file where POE's tracing and
2506 assertion messages should go. It's useful if you want the messages but
2507 have other plans for STDERR, which is where the messages go by default.
2508 POE's tests use this so the trace and assertion code can be
2510 instrumented during testing without spewing all over the terminal.
2511 Prefix: (none)
2513 Environment variable: POE_TRACE_FILENAME
2515 TRACE_FILES
2517 TRACE_FILES enables or disables traces in POE's filehandle watchers and
2518 the POE::Loop class that implements the lowest-level filehandle
2519 multiplexing. This may be useful when tracking down strange behavior
2520 related to filehandles.
2521 Prefix: <fh>
2523 Environment variable: POE_TRACE_FILES
2525 TRACE_REFCNT
2527 TRACE_REFCNT governs whether POE::Kernel will trace sessions' reference
2528 counts. As discussed in "Session Lifespans", POE does a lot of
2529 reference counting, and the current state of a session's reference
2530 counts determines whether the session lives or dies. It's common for
2531 developers to wonder why a session stops too early or remains active
2532 too long. TRACE_REFCNT can help explain why.
2533 Prefix: <rc>
2535 Environment variable: POE_TRACE_REFCNT
2537 TRACE_RETVALS
2539 TRACE_RETVALS can enable carping whenever a POE::Kernel method is about
2540 to fail. It's a non-fatal but noisier form of ASSERT_RETVALS.
2541 Prefix: <rv>
2543 Environment variable: POE_TRACE_RETVALS
2545 TRACE_SESSIONS
2547 TRACE_SESSIONS enables trace messages that pertain to session
2548 management. Notice will be given when sessions are created or
2549 destroyed, and when the parent or child status of a session changes.
2550 Prefix: <ss>
2552 Environment variable: POE_TRACE_SESSIONS
2554 TRACE_SIGNALS
2556 TRACE_SIGNALS turns on (or off) traces in POE's signal handling
2557 subsystem. Signal dispatch is one of POE's more complex parts, and the
2558 trace messages may help application developers understand signal
2559 propagation and timing.
2560 Prefix: <sg>
2562 Environment variable: POE_TRACE_SIGNALS
2564 USE_SIGCHLD
2566 Whether to use $SIG{CHLD} or to poll at an interval.
2567 This flag is enabled by default on Perl >= 5.8.1 as it has support for
2569 "safe signals". Please see perlipc for the gory details.
2570 You might want to disable this if you are running a version of Perl
2572 that is known to have bad signal handling, or if anything hijacks
2573 $SIG{CHLD}. One module that is known to do this is Apache.
2574 Enabling this flag will cause child reaping to happen almost
2576 immediately, as opposed to once per "CHILD_POLLING_INTERVAL".
2577 CHILD_POLLING_INTERVAL
2579 The interval at which "wait" is called to determine if child processes
2580 need to be reaped and the "CHLD" signal emulated.
2581 Defaults to 1 second.
2583 USE_SIGNAL_PIPE
2585 The only safe way to handle signals is to implement a shared-nothing
2586 model. POE builds a signal pipe that communicates between the signal
2587 handlers and the POE kernel loop in a safe and atomic manner. The
2588 signal pipe is implemented with POE::Pipe::OneWay, using a "pipe"
2589 conduit on Unix. Unfortunately, the signal pipe is not compatible with
2590 Windows and is not used on that platform.
2591 If you wish to revert to the previous unsafe signal behaviour, you must
2593 set "USE_SIGNAL_PIPE" to 0, or the environment variable
2594 "POE_USE_SIGNAL_PIPE".
2595 CATCH_EXCEPTIONS
2597 Whether or not POE should run event handler code in an eval { } and
2598 deliver the "DIE" signal on errors.
2599 See "Exception Handling".
2601
2603 POE's tests are lovely, dark and deep. These environment variables
2604 allow testers to take roads less traveled.
2605 POE_DANTIC
2607 Windows and Perls built for it tend to be poor at doing UNIXy things,
2608 although they do try. POE being very UNIXy itself must skip a lot of
2609 Windows tests. The POE_DANTIC environment variable will, when true,
2610 enable all these tests. It's intended to be used from time to time to
2611 see whether Windows has improved in some area.
2612
2614 The SEE ALSO section in POE contains a table of contents covering the
2615 entire POE distribution.
2616
2618 • There is no mechanism in place to prevent external reference count
2619 names from clashing.
2620 • There is no mechanism to catch exceptions generated in another
2622 session.
2623
2625 Please see POE for more information about authors and contributors.
2626perl v5.36.0 2022-07-22 POE::Kernel(3)